Solid electrolytic capacitor and manufacturing method thereof

ABSTRACT

A cathode foil and an anode foil, with an oxidized film layer formed on the surface and a separator interposed therebetween, are wound together to form a capacitor element, and prior to subjecting the capacitor element to chemical repair the content of the binder in the separator is adjusted to 10 to 20% with respect to the total weight of the separator. After the chemical repair, the capacitor element is immersed in mixed liquid prepared by mixing a polymerizable monomer and an oxidizing agent together with a predetermined solvent, a polymerization reaction of the electroconductive polymer is induced in the capacitor element, and a solid electrolyte layer is formed. The capacitor element is then inserted in an outer case; sealing rubber is mounted in the open-end portion and sealed with a tightening operation; and the unit is thereafter aged to form a solid electrolytic capacitor.

TECHNICAL FIELD

The present invention relates to a solid electrolytic capacitor and amanufacturing method thereof, and more particularly to a solidelectrolytic capacitor and manufacturing method thereof that reducesESR, and improves electrostatic capacity and withstand voltage. Thepresent invention also relates to a solid electrolytic capacitor andmanufacturing method thereof that improves the withstand voltage andinhibits LC fluctuation after re-flowing.

BACKGROUND ART

Electrolytic capacitors that use tantalum, aluminum, or another metalwith a valve action can attain a large capacity with a small size bygiving the valve action metal that serves as the anode-side counterelectrode the shape of a sintered body, an etching foil, or the like toexpand the surfaces of the dielectric, and are therefore widely used incommon practice. In addition to being small and having a large capacityand a low equivalent series resistance, solid electrolytic capacitors inwhich solid electrolytes are used as the electrolytes are, inparticular, easy to package on a chip, are suitable for surfacemounting, and posses other special characteristics, so these featuresare essential for miniaturizing, increasing the functionality, andlowering the costs of electronic equipment.

In this type of solid electrolytic capacitor, miniature and largecapacity applications commonly have a structure in which an anode foiland cathode foil composed of aluminum or another valve action metal are,with a separator interposed therebetween, wound together to form acapacitor element, the capacitor element is impregnated with a drivingelectrolytic solution, and the capacitor element is housed in a casecomposed of synthetic resin or in a case composed of aluminum or anothermetal and then sealed. Aluminum, as well as tantalum, niobium, titanium,and other metals are used as the anode material, and the same type ofmetal as the anode material is used as the cathode material.

The 7,7,8,8-tetracyanoquinodimethane (TCNQ) complex and Manganesedioxide are known as the solid electrolytes used in solid electrolyticcapacitors, but also available in recent years is technology (JapanesePatent Application Laid-open No. 2-15611) that features polyethylenedioxythiophene (hereinafter referred to as PEDT) or anotherelectro-conductive polymer that has a low reaction velocity andexcellent adhesion to the oxide film layer of an anodic electrode.

A solid electrolytic capacitor in which a solid electrolyte layercomposed of PEDT or another electro-conductive polymer is formed on sucha wound capacitor element is fabricated in the manner shown in FIG. 5.First, the surface of the anode foil composed of aluminum or anothervalve action metal is roughened by electrochemical etching in an aqueouschloride solution, a plurality of etching pits are formed, and voltageis thereafter applied to an aqueous solution of ammonium borate or thelike to form a dielectric oxide film layer (chemical conversion). In thesame manner as the anode foil, the cathode foil is also composed ofaluminum or another valve action metal, but the surface thereof issubjected to etching alone.

The anode foil on the surface of which an oxide film layer is formed andthe cathode foil on which etching pits alone are formed are woundtogether via an interposed separator to form a capacitor element. Next,a capacitor element that has been subjected to chemical repair issprayed with separately discharged 3,4-ethylene dioxythiophene(hereinafter referred to as EDT) or another polymerizable monomer, or isimpregnated with a mixed liquid of both, and polymerization reactionsare accelerated in the capacitor element to produce a solid electrolytelayer composed of PEDT or another electroconductive polymer. Thecapacitor element is thereafter encased in a cylindrical outer case witha closed end to fabricate a solid electrolytic capacitor.

In recent years, however, solid electrolytic capacitors as describedabove have come to be used in on-board equipment in vehicles. The drivevoltage for an on-board circuit is ordinarily 12V, and a high withstandvoltage of 25V is required in solid electrolytic capacitors. However,when manufacturing such a high withstand voltage product with aconventional manufacturing method such as that described above, there isa drawback in that the rate at which shorting occurs in the aging stepis high, and the yield is low.

High-melting lead-free solder has come to be used in recent years due toenvironmental concerns, and the solder reflow temperature has risen froma range of 200 to 220° C. to a range of 230 to 270° C. However,performing solder reflow under such high temperatures has a drawback inthat the withstand voltage is reduced. For this reason, a strong needexists for the development of a solid electrolytic capacitor whosewithstand voltage characteristics do not degrade even when hightemperature reflow soldering is carried out.

Such problems are not limited to the use of EDT as the polymerizablemonomer, and the same drawbacks occur when other thiophene derivatives,pyrrole, aniline, or the like are used.

A first object of the present invention is to provide a solidelectrolytic capacitor and a manufacturing method thereof that allow theESR to be reduced and the electro-static capacity to be improved.

A second object of the present invention is to provide a solidelectrolytic capacitor and a manufacturing method thereof that reducethe ESR and improve the electrostatic capacity and withstand voltage.

A third object of the present invention is to provide a solidelectrolytic capacitor and a manufacturing method thereof that canimprove the yield when manufacturing high withstand voltage products.

A fourth object of the present invention is to provide a solidelectrolytic capacitor and a manufacturing method thereof that canimprove withstand voltage and inhibit LC fluctuation after reflow.

DISCLOSURE OF THE INVENTION

The present invention is a solid electrolytic capacitor in which aseparator containing a compound with a vinyl group is used, wherein theESR is reduced and electrostatic capacity is improved by suitablyadjusting the content of the compound with a vinyl group in theseparator.

In a solid electrolytic capacitor in which a separator containing acompound with a vinyl group is used, the ESR can be reduced and theelectrostatic capacity and withstand voltage improved by adding apredetermined coupling agent and a surfactant to the capacitor elementprior to impregnation with a polymerizable monomer and oxidizing agent.

Furthermore, in a solid electrolytic capacitor in which a separatorcontaining a compound with a vinyl group is used, degradation of thewithstand voltage characteristics due to lead-free reflow can beprevented and the yield when manufacturing a high withstand voltageproduct can be improved by impregnating the capacitor element with aborate compound or another additive solution, forming a conjugatecomposed of a compound having a vinyl group and a borate compound oranother additive, and thereafter forming a solid electrolytic layercomposed of an electroconductive polymer.

In a solid electrolytic capacitor in which a separator containing acompound with a vinyl group is used, the withstand voltage is improvedand LC fluctuation after reflow can be inhibited by impregnating acapacitor element with a polyimide silicon solution, and forming a filmcomposed of polyimide silicon and a compound with a vinyl group.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the structural formula of a thiophenederivative;

FIGS. 2 to 4 are flowcharts showing examples of the manufacturing stepfor a solid electrolytic capacitor related to the present invention; and

FIG. 5 is a flowchart showing an example of the manufacturing step for asolid electrolytic capacitor using conventional technology.

BEST MODE FOR CARRYING OUT THE INVENTION (A) First Embodiment

The present inventors, as a result of thoroughgoing research to achievethe first object, discovered that the ESR is reduced and theelectrostatic capacity is improved by configuring the binder of theseparator with a compound having a vinyl group, and making this binder10 to 20% of the total weight of the separator.

In other words, the present inventors conducted studies while varyingthe content of the binder composed of a compound with a vinyl group toconduct the investigation, and discovered that when the above-describedrange is exceeded, ESR and the electrostatic capacity characteristicsare reduced, and that the characteristics remain constant when thecontent is less than this range.

(A-1) Method for Manufacturing the Solid Electrolytic Capacitor

The method for manufacturing the solid electrolytic capacitor in thefirst embodiment is described below. That is, a cathode foil and ananode foil, with an oxidized film layer formed on the surface and aseparator interposed therebetween, are wound together to form acapacitor element, and prior to subjecting the capacitor element tochemical repair the content of the binder in the separator is adjustedto 10 to 20% with respect to the total weight of the separator.

After the chemical repair, the capacitor element is immersed in mixedliquid prepared by mixing a polymerizable monomer and an oxidizing agenttogether with a predetermined solvent, a polymerization reaction of theelectroconductive polymer is induced in the capacitor element, and asolid electrolyte layer is formed. The capacitor element is theninserted in an outer case; sealing rubber is mounted in the open-endportion and sealed with a tightening operation; and the unit isthereafter aged to form a solid electrolytic capacitor.

(A-2) Separator

A separator for a solid electrolytic capacitor principally composed ofsynthetic resin is ordinarily composed of a binder that joins thesynthetic resin and other components together. The synthetic resinitself may be used as the binder, or the synthetic resin may be madeinto a fibrous form and melted in the separator fabrication process toform the main fiber.

It was discovered in the first embodiment that adequate results can beobtained when a separator is used in which a compound with a vinyl groupis used as a binder, and the content of the binder in the separatorprior to chemical repair is 10 to 20%, and is more preferably 13 to 17%,with respect to the total weight of the separator.

Here, polyvinyl alcohol (PVA), polyvinyl acetate, polyvinyl pyrrolidone,polyacrylamide, or the like may be used as the compound with a vinylgroup, but among these PVA is preferred. Polyester fiber or nylon fiberwith excellent thermal resistance is preferred as the main fiber of theseparator. A separator composed of vinylon fiber is not suitable becauselead-free reflow characteristics cannot be satisfied.

Described next is a method for adjusting the content of the binder inthe separator prior to chemical repair to the above-stated range. Aseparator containing 10% or more of a binder composed of a compound witha vinyl group is obtained in which the binder in the separator isdissolved with a hot water immersion treatment and adjusted to theabove-described range.

In the case of hot water immersion treatment, the temperature of the hotwater is preferably 60 to 100° C. at which PVA and the like dissolve,and the hot water immersion treatment time is preferably 5 to 180minutes, but is more preferably 20 to 60 minutes. The hot waterimmersion treatment is preferably performed with running water becausePVA or the like dissolves faster and more uniformly.

This hot water immersion treatment is preferably performed after thecapacitor element has been formed. When the hot water immersiontreatment is performed prior to the formation of the capacitor element,the strength of the separator is reduced, so the winding strength duringwinding is reduced, the formation state of the electroconductive polymerin the capacitor element is worsened, and the characteristics aredegraded.

(A-3) EDT and Oxidizing Agent

When EDT is used as the polymerizable monomer, an EDT monomer can beused as the EDT with which the capacitor element is impregnated, but amonomer solution in which EDT and a volatile solvent are mixed in avolume ratio of 1:0 to 1:3 may also be used.

Pentane or another hydrocarbon, tetrahydrofuran or another ether, ethylformate or another ester, acetone or another ketone, methanol or anotheralcohol, or acetonitrile or another nitrogen compound may be used as thevolatile solvent, but preferably used among these is methanol, ethanol,acetone, or the like.

Ferric p-toluenesulfonate dissolved in ethanol, or an aqueous solutionof periodic acid or iodic acid can be used as the oxidizing agent, andthe concentration of the oxidizing agent with respect to the solvent ispreferably 40 to 58 wt %, and is more preferably 45 to 57 wt %. The ESRdecreases with increased concentration of the oxidizing agent withrespect to the solvent. A volatile solvent used for the above-describedmonomer solutions may be used as the solvent for the oxidizing agent,and among these ethanol is advantageous. The reason that ethanol isadvantageous as the solvent for the oxidizing agent is believed to bethat the vapor pressure is low, so evaporation easily occurs, and theremaining amount is small.

(A-4) Chemical Conversion Solution for Chemical Repair

Ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or anotherphosphate-based chemical conversion solution; ammonium borate or anotherboric acid-based chemical conversion solution; or ammonium adipate oranother adipic acid-based chemical conversion solution may be used asthe chemical conversion solution for chemical repair, but preferablyused among these is ammonium dihydrogen phosphate. The immersion time ispreferably 5 to 120 minutes.

(A-5) Other Polymerizable Monomers

The polymerizable monomer used in the present invention may be, inaddition to the above-described EDT, a thiophene derivative, aniline,pyrrole, furan, acetylene, or a derivative thereof other than EDT, aslong as oxidation polymerization is carried out with a predeterminedoxidizing agent to form a polymerizable monomer. Substances with thestructural formula shown in FIG. 1 may be used for the thiophenederivative.

(A-6) Operation and Effect of the First Embodiment

Following are believed to be the reasons that adequate results can beobtained by using a compound having a vinyl group as a binder for theseparator and maintaining the content of the binder prior to chemicalrepair at 10 to 20% with respect to the total weight of the separator,as described above. In other words, constant amounts were obtained whena comparison was made between the amounts in which the resulting PEDT orother electroconductive polymer was retained in a separator in which thebinder content was in the indicated range, and a separator in which thebinder content exceeded the indicated range. When a separator in whichthe binder content is in this range is used, the characteristics of thesolid electrolytic capacitor are improved, and when a separator in whichthe binder content exceeds this range is used, the characteristics ofthe solid electrolytic capacitor are degraded.

Due to this fact, it is possible that when a large quantity of acompound with a vinyl group is present as a binder component in aprocess in which PEDT or another electroconductive polymer is used, thiscompound negatively affects the formation of PEDT, and the inherentcharacteristics (electrostatic capacity, ESR) of PEDT or anotherelectroconductive polymer cannot be obtained. It is thought thatadequate results were obtained by suitably adjusting the amount of thecompound with a vinyl group as a binder component.

(A-7) Examples Related to the First Embodiment

The invention of the first embodiment is described next on the basis ofExamples A1 to A5 and Comparative Example A1 manufactured in the mannerdescribed below.

Example A1

PET fiber was used as the main fiber, and a solid electrolytic capacitorwas fabricated as follows by using a separator containing 15% of abinder composed of PVA. An electrode-forming device was connected to thecathode foil and anode foil on whose surface an oxide film layer hadbeen formed, and both electrode foils were wound with the interposedseparator to form a capacitor element whose element shape was 5φ×2.8 L.This capacitor element was immersed for 20 minutes in hot water at atemperature of 100° C., and the binder content was adjusted to 13% afterthe hot water immersion treatment. The capacitor element was immersedfor 40 minutes in ammonium dihydrogen phosphate, and chemical repair wascarried out.

EDT and an ethanol solution of 45% ferric p-toluenesulfonate were mixedin a container, the capacitor element was immersed for 10 seconds in themixed liquid and heated for 60 minutes at 120° C., and a PEDTpolymerization reaction was induced in the capacitor element to form asolid electrolyte layer.

The capacitor element was inserted into a cylindrical outer case with aclosed end, and sealing rubber was mounted in the open-end portion andsealed with a tightening operation. Aging was thereafter carried out for120 minutes at 150° C. with an applied voltage of 8.2 V to form a solidelectrolytic capacitor. The rated voltage of the solid electrolyticcapacitor was 6.3 WV, and the rated capacity was 120 μF.

Example A2

Using a separator containing 25% of a binder composed of PVA, acapacitor element was formed and immersed for 20 minutes in hot water ata temperature of 100° C., and after the hot water immersion treatmentthe binder content was adjusted to 14%. Other conditions and steps werethe same as Example A1.

Example A3

Using a separator containing 30% of a binder composed of PVA, acapacitor element was formed and immersed for 10 minutes in hot water ata temperature of 100° C., and after the hot water immersion treatmentthe binder content was adjusted to 20%. Other conditions and steps werethe same as Example A1.

Example A4

Using a separator containing 30% of a binder composed of PVA, acapacitor element was formed and immersed for 20 minutes in hot water ata temperature of 100° C., and after the hot water immersion treatmentthe binder content was adjusted to 15%. Other conditions and steps werethe same as Example A1.

Comparative Example A1

Using a separator containing 30% of a binder composed of PVA, chemicalrepair was carried out without hot water immersion treatment. Otherconditions and steps were the same as Example A1.

[Comparison of the Results]

The initial characteristics of the Examples A1 to A5 and ComparativeExample A1 carried out as described above were investigated and theresults shown in Table 1 were obtained. TABLE 1 CONTENT OF IMMERSIONBINDER AFTER INITIAL CONTENT TIME IN HOT IMMERSION CHARACTERISTICS OFBINDER WATER TREATMENT CAP ESR (%) (MINUTES) (%) (μF) (Ω/100 kHz)EXAMPLE A1 15 20 13 141 0.0185 EXAMPLE A2 25 20 14 139 0.0185 EXAMPLE A330 10 20 130 0.0200 EXAMPLE A4 30 20 15 138 0.0186 COMPARATIVE 30 — 30120 0.0215 EXAMPLE A1

As a result of comparing Examples A3 and A4 with Comparative Example A1in which a separator containing 30% binder composed of PVA was used, itis apparent from Table 1 that both the electrostatic capacity and ESR ofExamples A3 and A4 were superior to Comparative Example A1 in which hotwater immersion was not carried out. The results furthermore indicatethat both the electrostatic capacity and ESR of Example A4, in which thehot water immersion time was longer, were superior to Example A3.

(C) Second Embodiment

The present inventors, as a result of thoroughgoing research to achievethe second object, succeeded in obtaining a solid electrolytic capacitorand a manufacturing method thereof that can reduce the ESR and improvethe electrostatic capacity and withstand voltage by using a separatorthat contains as a binder 10 wt % or more of a compound having a vinylgroup, and adding one or two compounds selected from acetylene diol anddimethyl lauryl amine oxide to the capacitor element prior toimpregnation with a polymerizable monomer and an oxidant.

(C-1) Method for Manufacturing the Solid Electrolytic Capacitor

The method for manufacturing the solid electrolytic capacitor in thesecond embodiment is described below. That is, a cathode foil and ananode foil, with an oxidized film layer formed on the surface and aseparator that contains as a binder 10 wt % or more of a compound havinga vinyl group interposed therebetween, are wound together to form acapacitor element, and the capacitor element is subjected to chemicalrepair. After adding to the capacitor element one or two compoundsselected from acetylene diol and dimethyl lauryl amine oxide, thecapacitor element is immersed in a mixed liquid of a polymerizablemonomer and an oxidizing agent, and a polymerization reaction involvingthe electroconductive polymer is induced in the capacitor element toform a solid electrolyte layer. The capacitor element is then insertedin an outer case; sealing rubber is mounted in the open-end portion andsealed with a tightening operation; and the unit is thereafter aged toform a solid electrolytic capacitor.

(C-2) Separator

A separator for a solid electrolytic capacitor principally composed ofsynthetic resin is ordinarily composed of a binder that joins thesynthetic resin and other components together. The synthetic resinitself may be used as the binder, or the synthetic resin may be madeinto a fibrous form and melted in the separator fabrication process toform the main fiber. Used in the present invention is a separator inwhich a compound with a vinyl group is used as a binder.

Here, polyvinyl alcohol (PVA), polyvinyl acetate, polyvinyl pyrrolidone,polyacrylamide or the like may be used as the compound with a vinylgroup, but among these PVA is preferred.

The use of polyester fiber or nylon fiber with excellent thermalresistance as the main fiber of the separator is advantageous in thatthermal resistance is improved. The content of the binder composed of acompound with a vinyl group is preferably 10 wt % or more with respectto the separator. When the content is less than 10 wt %, sufficienteffect cannot be obtained.

(C-3) Additives

It was found that when one or two compounds selected from acetylene dioland dimethyl lauryl amine oxide as surfactants are added as additives tothe capacitor element, the electrostatic capacity increases and the ESRdecreases. It is thought that the reason for this is that theadhesiveness of PVA and PEDT is increased.

A preferred method for adding these additives to the capacitor elementis to carry out chemical repair, immerse the capacitor element at normaltemperature in 0.1 to 10%, and more preferably 0.5 to 2% aqueoussolution, dry the capacitor element at 50 to 90° C., and thereaftercarry out heating treatment at 150 to 200° C. It is thought that thisheating improves the bridged state of the PVA, PEDT, and surfactant, andincreases the adhesiveness.

(C-4) EDT and Oxidizing Agent

When EDT is used as the polymerizable monomer, an EDT monomer can beused as the EDT with which the capacitor element is impregnated, but amonomer solution in which EDT and a volatile solvent are mixed at avolume ratio of 1:0 to 1:3 may also be used.

Pentane or another hydrocarbon, tetrahydrofuran or another ether, ethylformate or another ester, acetone or another ketone, methanol or anotheralcohol, or acetonitrile or another nitrogen compound may be used as thevolatile solvent, but preferably used among these is methanol, ethanol,acetone, or the like.

Ferric p-toluenesulfonate dissolved in ethanol, or an aqueous solutionof periodic acid or iodic acid can be used as the oxidizing agent, butthe concentration of the oxidizing agent with respect to the solvent ispreferably 40 to 65 wt %, and is more preferably 45 to 57 wt %. The ESRdecreases with increased concentration of the oxidizing agent withrespect to the solvent. A volatile solvent used for the above-describedmonomer solutions may be used as the solvent for the oxidizing agent,and among these ethanol is advantageous. The reason that ethanol isadvantageous as the solvent for the oxidizing agent is believed to bethat the vapor pressure is low, so evaporation easily occurs, and theremaining amount is small.

(C-5) Chemical Conversion Solution for Chemical Repair

Ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or anotherphosphate-based chemical conversion solution; ammonium borate or anotherboric acid-based chemical conversion solution; or ammonium adipate oranother adipic acid-based chemical conversion solution may be used asthe chemical conversion solution for chemical repair, but preferablyused among these is ammonium dihydrogen phosphate. The immersion time ispreferably 5 to 120 minutes.

(C-6) Other Polymerizable Monomers

The polymerizable monomer used in the present invention may be, inaddition to the above-described EDT, a thiophene derivative, aniline,pyrrole, furan, acetylene, or a derivative thereof other than EDT, aslong as oxidation polymerization is carried out with a predeterminedoxidizing agent to form a polymerizable monomer. Substances with thestructural formula shown in FIG. 1 may be used for the thiophenederivative.

(C-7) Operation and Effect of the Second Embodiment

The reason that the ESR is reduced and an improvement in theelectrostatic capacity and withstand voltage can be obtained with theconfiguration of the above-described second embodiment is that when PVAdissolved during chemical repair adheres to the dielectric film,additives are added thereafter, and PEDT is then formed, theadhesiveness of the PVA and PEDT is increased and the electrostaticcapacity and ESR are improved.

Furthermore, PVA elutes from the separator during chemical repair andthe voids in the separator increase in size, so more PEDT is retained inthe separator and the ESR decreases. The attack of the PVA deposited onthe dielectric film toward the dielectric film of oxidizing agentremaining in the PEDT is reduced and the withstand voltage is increased.If the PVA content is less than 10 wt %, these effects are reduced.

(C-8) Examples Related to the Second Embodiment

The invention of the second embodiment is described next on the basis ofExamples and conventional examples manufactured in the manner describedbelow.

Example C1

PET fiber was used as the main fiber, and a solid electrolytic capacitorwas fabricated as follows by using a separator containing 10 wt % of PVAas a binder. An electrode-forming device was connected to the cathodefoil and the anode foil on whose surface an oxide film layer had beenformed, and both electrode foils were wound with the interposedseparator to form a capacitor element. The capacitor element wasimmersed for 40 minutes in ammonium dihydrogen phosphate, and chemicalrepair was carried out. After the chemical repair, the capacitor elementwas dried at 100° C., immersed for three minutes at normal temperaturein an aqueous solution of 1 wt % acetylene diol, dried at 60° C., andheated at 170° C.

EDT and an ethanol solution of 45% ferric p-toluenesulfonate were mixedin a container, the capacitor element was immersed for 10 seconds in themixed liquid and heated for 60 minutes at 120° C., and a PEDTpolymerization reaction was induced in the capacitor element to form asolid electrolyte layer. The capacitor element was inserted into acylindrical outer case with a closed end, and sealing rubber was mountedin the open-end portion and sealed with a tightening operation. Agingwas thereafter carried out for 120 minutes at 150° C. with an appliedvoltage of 5.2 V to form a solid electrolytic capacitor. The ratedvoltage of the solid electrolytic capacitor was 4 WV, and the ratedcapacity was 180 μF.

Example C2

Chemical repair was carried out, the capacitor element was then dried at100° C., thereafter immersed for three minutes at normal temperature inan aqueous solution of 1 wt % dimethyl lauryl amine oxide, dried at 60°C., and heated at 170° C. Other conditions and steps were the same asExample C1.

Comparative Example C1

Additives were not added to the capacitor element, but other conditionsand steps were kept the same as Example C1 to fabricate a solidelectrolytic capacitor.

[Comparison of the Results]

The initial characteristics of the Examples C1 and C2, and ComparativeExample C1 carried out as described above were investigated and theresults shown in Table 2 were obtained. TABLE 2 INITIAL CHARACTERISTICSCAP ESR ADDITIVE (μF) (Ω/100 kHz) EXAMPLE C1 ACETYLENE DIOL 80 18.5EXAMPLE C2 DIMETHYL LAURYL 75 19.0 AMINE OXIDE COMPARATIVE NOT USED 7120.5 EXAMPLE C1

It is apparent from Table 2 that Examples C1 and C2 exhibit a greaterelectrostatic capacity and a lower ESR than Comparative Example C1.

When a comparison was made of the use of a separator in which PVA wasnot included as a binder, the shorting voltage increased 20V.

(D) Third Embodiment

The present inventors, arrived at the following conclusions as a resultof thoroughgoing research as to the cause of the higher ratio ofshorting occurrences in the aging step when manufacturing a highwithstand voltage product in order to achieve the third object.Specifically, in addition to an electroconductive polymer, also presentin the capacitor after the electroconductive polymer has been formed aremonomers and oxidizing agents unrelated to the polymerization reaction,and other reaction residues. The withstand voltage of the substancesother than the electroconductive polymer is lower than the withstandvoltage of the electroconductive polymer, so it is thought that thesesubstances reduce the withstand voltage of the solid electrolyticcapacitor.

In view of the above, the present inventors, as a result ofthoroughgoing research to improve the withstand voltage of the solidelectrolytic capacitor and to prevent degradation of the withstandvoltage characteristics due to lead-free reflow in the presence of thesereaction residues, found that the withstand voltage of the solidelectrolytic capacitor can be improved by forming a capacitor elementusing a separator containing a compound with a vinyl group and adding aborate compound to the capacitor element.

(D-1) Method for Manufacturing the Solid Electrolytic Capacitor

The method for manufacturing the solid electrolytic capacitor in thethird embodiment is described below. That is, a cathode foil and ananode foil, with an oxidized film layer formed on the surface and aseparator containing a compound with a vinyl group interposedtherebetween, are wound together to form a capacitor element, and thecapacitor element is subjected to chemical repair. Next, a solution of aborate compound is impregnated and heated in the capacitor element, aconjugate composed of a compound with a vinyl group and a boratecompound is generated, the capacitor element is thereafter immersed in amixed liquid that was prepared by mixing a polymerizable monomer and anoxidizing agent together with a predetermined solvent, and apolymerization reaction involving the electroconductive polymer isinduced in the capacitor element to form a solid electrolyte layer. Thecapacitor element is then inserted in an outer case; sealing rubber ismounted in the open-end portion and sealed with a tightening operation;and the unit is thereafter aged to form a solid electrolytic capacitor.It is even more advantageous to add a predetermined coupling agent tothe capacitor element prior to impregnation with the polymerizablemonomer and the oxidizing agent.

(D-2) Separator

A separator for a solid electrolytic capacitor principally composed ofsynthetic resin is ordinarily composed of a binder that joins thesynthetic resin and other components together. The synthetic resinitself may be used as the binder, or the synthetic resin may be madeinto a fibrous form and melted in the separator fabrication process toform the main fiber. Adequate results were obtained in the presentinvention by using a separator in which a compound with a vinyl group isused as the main fiber or binder of such a separator.

The required amount of compound with a vinyl group that is added to themain fiber or binder of the separator may be a small amount, but theeffect is not diminished with a larger amount. The reason for this isthat the compound with a vinyl group that was added to the separatorelutes and adheres to the oxide film layer, so the effect of the presentinvention can be obtained. Therefore, the separator may be formed with100% vinylon fiber in the same manner as a vinylon separator. In thiscase, the elution amount should be controlled so that an excessiveamount of compound with a vinyl group is not eluted in the manufacturingstep, reducing the strength of the separator. A typical example of thepresent invention is a separator that uses a PVA binder, but in thiscase, the content thereof is preferably kept at 10 to 20 wt % in orderto obtain a predetermined strength.

Examples of compounds with a vinyl group that are preferably used inthis case are polyvinyl alcohol (hereinafter abbreviated as PVA),polyvinyl acetate, polyvinyl pyrrolidone, and polyacrylamide, but PVA ispreferred. More specifically, PVA fiber (vinylon) or undrawn vinylon maybe used as the main fiber of the separator, and a PVA polymer or undrawnvinylon may be used as the binder. Vinylon fiber with a fiber diameterof 3.0 to 12.0 μm is cut into short fibers with a predetermined cutlength, and, using a predetermined binder, unwoven cloth obtained withany device may be used, for example.

Methods that may be used for adding a compound with a vinyl group to theseparator include a method in which the separator is composed of thebinder or main fiber as described above (that is to say, a method foradding the compound with a vinyl group as a constituent component of theseparator), a method in which the separator is immersed in a solution ofthe compound with a vinyl group, and a method for applying the compoundwith a vinyl group.

(D-3) Borate Compound

Examples of borate compounds that may be used include boric acid, borax,ammonium salt of boric acid, metal salt or another metal salt, andtriethyl borate or another ester of boric acid, but boric acid ispreferred.

The solvent for these borate compounds should be one in which thesecompounds are dissolved, and mainly water, glycerin, or the like can beused. The concentration of the borate compound solution is preferably0.1 wt % to 10 wt %, and is more preferably 3 wt % to 7 wt %. When theconcentration of the borate compound solution is not in this range, theeffect is reduced. This is due to the fact that when the concentrationof borate compound solution is less than 0.1 wt %, the amount ofconjugate formed is insufficient because the amount of borate compoundin the solution is small. Although the reason is not apparent, when theconcentration of borate compound solution is greater than 10 wt %, theexcess boric acid exerts a negative effect after the conjugate isformed, and the ESR increases.

(D-4) Method for Adding the Borate Compound to the Capacitor Element

A method of immersing a capacitor element in the borate compoundsolution, or a method for discharging the borate compound solution overthe capacitor element may be used for adding the borate compound to thecapacitor element.

It was found that when the borate compound is added to the capacitorelement and heating treatment is performed thereafter, the initialcharacteristics improve. It is thought that the reason for this is thatthe compound with a vinyl group contained in the separator elutes intothe capacitor element and the hydrophobicity of the end groups thereofincreases, so the adhesiveness of the oxide film and the solidelectrolyte improves. The heating temperature is preferably 120 to 250°C., and is more preferably 150 to 200° C. When the heating temperatureis not in this range, the effect is reduced. This is thought to be dueto the fact that when the heating temperature is less than 120° C.,reactions such as the one that increases the hydrophobicity of the endgroups of the compound with a vinyl group do not adequately progress,and when the heating temperature exceeds 250° C., the thermaldegradation of the compound with a vinyl group takes place and theeffect is reduced.

(D-5) Timing for Adding the Borate Compound in the Capacitor Element

The present inventors thoroughly researched the timing for adding theborate compound to the capacitor element. As a result, it becameapparent that the addition may be made at a stage prior to the step forforming an electroconductive polymer, or at any other stage. In otherwords, the timing may be prior to chemical repair, as described above,or the compound may be allowed to deposit on the electrode foils priorto forming the capacitor element, and the methods (1) to (3) describedbelow, for example, may be considered. The method in (1) corresponds tothe manufacturing method described in (D-1).

The most advantageous of the methods (1) to (3) described below ismethod (1), in which an anode foil and cathode foil are wound while aseparator to which a compound with a vinyl group has been added isinterposed therebetween to form a capacitor element, the capacitorelement is subjected to chemical repair, the capacitor element is thenimpregnated with a solution of borate compound, a conjugate composed ofa borate compound and a compound with a vinyl group is formed, and apolymerization reaction involving the electroconductive polymer isthereafter induced in the capacitor element to form a solid electrolytelayer. The effect of the present invention is not changed if the resinsealing is not performed with the methods described below.

(1) After Chemical Repair . . . Refer to FIG. 2

This method is carried out in the following order: chemical conversion,capacitor element formation using a separator containing a compound witha vinyl group, chemical repair, immersion in a solution of a boratecompound, impregnation with a polymerizable monomer and an oxidizingagent, polymerization, insertion in an outer case, resin sealing, andaging.

(2) After Capacitor Element Formation, but Prior to Chemical Repair . .. Refer to FIG. 3

This method is carried out in the following order: chemical conversion,capacitor element formation using a separator containing a compound witha vinyl group, immersion in a solution of a borate compound, chemicalrepair, impregnation with a polymerizable monomer and an oxidizingagent, polymerization, insertion in an outer case, resin sealing, andaging.

(3) Prior to Capacitor Element Formation . . . Refer to FIG. 4

This method is carried out in the following order: chemical conversion,immersion of at least one of the electrode foils in a solution of aborate compound (or application and drying thereafter), capacitorelement formation using a separator containing a compound with a vinylgroup, chemical repair, impregnation with a polymerizable monomer and anoxidizing agent, polymerization, insertion in an outer case, resinsealing, and aging.

The concentration of the solution of a borate compound, temperature,impregnation time, drying temperature, drying time, and other parametersin these methods are the same as described above.

(D-6) Coupling Agent

The following silane coupling agents, titanium coupling agents, andaluminum coupling agents may be used for the coupling agent, and one, ortwo or more coupling agents selected from among these may be used.

Methods that entail performing chemical repair, then immersing thecapacitor element for several minutes in an aqueous solution of acoupling agent with a concentration of 0.1 to 5%, and more preferably0.5 to 3%, removing the capacitor element from the solution, andthereafter drying the capacitor element at 50 to 100° C. may be used foradding these coupling agents to the capacitor element.

Examples of silane coupling agents include: vinyl trichlorosilane,vinyl(β-methoxysilane) vinyl triethoxysilane, vinyl trimethoxysilane,vinyl tris(β-methoxyethoxy)silane, γ-methacryloxysilane,γ-methacryloxypropyl methyl dimethoxy silane, γ-methacryloxypropyltrimethoxysilane, β-(3,4-epoxycyclohexyl)ethyl trimethoxysilane,γ-glycidoxypropyl trimethoxysilane, γ-glycidoxypropyl methyldiethoxysilane, N-β-(aminoethyl)-γ-aminopropyl methoxysilane,γ-aminopropyl triethoxysilane, N-phenyl-γ-aminopropyl methoxysilane,γ-mercaptopropyl methoxysilane, and γ-chloropropyl methoxysilane.

Examples of titanium coupling agents include: isopropyl triisostearoyltitanate, isopropyl tris(dioctyl pyrophosphite) titanate, isopropyltri(N-aminoethyl-aminoethyl)titanate, isopropyl tridecyl benzenesulfonyl titanate, tetraoctyl bis(ditridecyl phosphite)titanate,tetra(2,2-diaryloxymethyl-1-butyl)-bis(di-tridecyl)phosphite titanate,bis(dioctyl pyrophosphate)oxyacetate titanate, bis(dioctylpyrophosphate)ethylene titanate, isopropyl trioctanoyl titanate,isopropyl dimethacryl isostearoyl titanate, and isopropyl tricumylphenyl titanate.

An example of the aluminum coupling agent is an acetal alkoxy aluminumdiisopropylate.

(D-7) EDT and Oxidizing Agent

When EDT is used as the polymerizable monomer, an EDT monomer can beused as the EDT with which the capacitor element is impregnated, but amonomer solution in which EDT and a volatile solvent are mixed at avolume ratio of 1:0 to 1:3 may also be used.

Pentane or another hydrocarbon, tetrahydrofuran or another ether, ethylformate or another ester, acetone or another ketone, methanol or anotheralcohol, or acetonitrile or another nitrogen compound may be used as thevolatile solvent, but preferably used among these is methanol, ethanol,acetone, or the like.

Ferric p-toluenesulfonate dissolved in ethanol, or an aqueous solutionof periodic acid or iodic acid can be used as the oxidizing agent, butthe concentration of the oxidizing agent with respect to the solvent ispreferably 40 to 57 wt %, and is more preferably 45 to 57 wt %. The ESRdecreases with increased concentration of the oxidizing agent withrespect to the solvent. A volatile solvent used for the above-describedmonomer solutions may be used as the solvent for the oxidizing agent,and among these ethanol is advantageous. The reason that ethanol isadvantageous as the solvent for the oxidizing agent is believed to bethat the vapor pressure is low, so evaporation easily occurs, and theremaining amount is small.

(D-8) Reduced Pressure

It is even more preferable to reduce the pressure in the polymerizationstep. The reason for this is that when the pressure is reduced duringheat polymerization, residual matter can be evaporated away togetherwith polymerization. The pressure is preferably reduced to about 10 to360 mmHg.

(D-9) Impregnation Step

The time for impregnating the capacitor element with the mixed liquid isdetermined by the size of the capacitor element, but 5 seconds or moreis preferred for a capacitor element with a size of about φ 5×3 L, and10 seconds or more is preferred for a capacitor element with a size ofabout φ 9×5 L. Impregnation for 5 seconds is required at minimum. Thereare no drawbacks to the characteristics even if impregnation is carriedout over a long period of time.

It is advantageous to keep the pressure reduced after impregnation hasbeen performed in this manner. It is thought that the reason for this isthat the residual amount of volatile solvent is reduced. The reducedpressure conditions are the same as the reduced pressure conditions inthe above-described polymerization step.

(D-10) Chemical Conversion Solution for Chemical Repair

Ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or anotherphosphate-based chemical conversion solution; ammonium borate or anotherboric acid-based chemical conversion solution; or ammonium adipate oranother adipic acid-based chemical conversion solution may be used asthe chemical conversion solution for chemical repair, but preferablyused among these is ammonium dihydrogen phosphate. The immersion time ispreferably 5 to 120 minutes.

(D-11) Other Polymerizable Monomers

The polymerizable monomer used in the present invention may be, inaddition to the above-described EDT, a thiophene derivative, aniline,pyrrole, furan, acetylene, or a derivative thereof other than EDT, aslong as oxidation polymerization is carried out with a predeterminedoxidizing agent to form a polymerizable monomer. Substances with thestructural formula shown in FIG. 1 may be used for the thiophenederivative.

(D-12) Operation and Effect of the Third Embodiment

As described above, degradation of the withstand voltage characteristicsdue to lead-free reflow can be prevented, and the ratio of shortingoccurrence in the aging step can be considerably reduced by forming acapacitor element using a separator containing a compound with a vinylgroup, and adding a borate compound to the capacitor element at apredetermined time.

It is thought that the reason that such effects can be obtained is thatby allowing the compound with a vinyl group contained in the separatorto elute into the capacitor element to form a conjugate with a boratecompound or another hydrogen bond in the capacitor element, and allowingthis conjugate to deposit on the oxide film of the electrode foils toform a layer, the adhesiveness of the oxide film and the solidelectrolyte improves, and because the withstand voltage of this layer ishigh, the withstand voltage of the capacitor is also improved.

Allowing the compound with a vinyl group added to the separator to eluteas in the present invention is more advantageous than adding a compoundwith a vinyl group to the element after formation of the capacitorelement in that the conjugate with borate compound uniformly deposits onthe oxide film.

In particular, it is thought that when PVA and boric acid are used, aconjugate composed of an ester compound is formed, and this estercompound deposits on the surface of the film to form an adequate layerwithout immersion in a dielectric film, so adequate characteristics canbe obtained.

As described above, it is thought that when heating treatment is carriedout after adding the borate compound, the bonding characteristics of theend group of the compound with a vinyl group eluted from the separatorwith respect to the dielectric oxide layer and the electroconductivepolymer are improved, and the initial characteristics, and theelectrostatic capacity and ESR characteristics in particular, areenhanced.

Also, when a coupling agent is added to the capacitor element in thethird embodiment, adhesion to PEDT is further enhanced by thesynergistic effect with the conjugate, and the electrostatic capacityand ESR are improved.

When the PVA content is less than 10 wt %, these effects are reduced.

(D-13) Examples Related to the Third Embodiment

The invention of the third embodiment is described next on the basis ofExamples, Comparative Examples, and conventional examples manufacturedin the manner described below.

Example D1

A solid electrolytic capacitor was fabricated as follows by using aseparator containing a PVA polymer as a binder and containing 49 wt % ofPET fiber as the main fiber. An electrode-forming device was connectedto the cathode foil and the anode foil on whose surface an oxide filmlayer had been formed, and both electrode foils were wound with theinterposed separator to form a capacitor element whose element shape was5φ×2.8 L. The capacitor element was immersed for 40 minutes in ammoniumdihydrogen phosphate, and chemical repair was carried out. After thechemical repair, the capacitor element was dried at 100° C., immersedfor three minutes at normal temperature in an aqueous solution of 5 wt %boric acid, and heated at 175° C.

EDT and an ethanol solution of 45% ferric p-toluenesulfonate were mixedin a container, and the capacitor element was immersed for 10 seconds inthe mixed liquid, held in a state of reduced pressure of about 250 mmHg,and subsequently heated for 60 minutes at 120° C. under the sameconditions. A PEDT polymerization reaction was induced in the capacitorelement to form a solid electrolyte layer.

The capacitor element was inserted into a cylindrical outer case with aclosed end, and sealing rubber was mounted in the open-end portion andsealed with a tightening operation. Aging was thereafter carried out for120 minutes at 150° C. with an applied voltage of 33 V to form a solidelectrolytic capacitor. The rated voltage of the solid electrolyticcapacitor was 25 WV, and the rated capacity was 15 μF.

Example D2

PET fiber was used as the main fiber, and a solid electrolytic capacitorwas fabricated as follows using a separator containing 10 wt % of PVA asa binder. An electrode-forming device was connected to the cathode foiland the anode foil on whose surface an oxide film layer had been formed,and both electrode foils were wound with the interposed separator toform a capacitor element. The capacitor element was immersed for 40minutes in ammonium dihydrogen phosphate, and chemical repair wascarried out. After the chemical repair, the capacitor element was driedat 100° C., then immersed for several minutes at normal temperature inan aqueous solution of 5 wt % boric acid, and dried for one hour at 150°C. The capacitor element was also immersed for one minute at normaltemperature in an aqueous solution of 1 wt % N-β(aminoethyl)γ-aminopropyl methoxy silane, and dried for one hour at 80° C.

EDT and an ethanol solution of 45% ferric p-toluenesulfonate were mixedin a container, the capacitor element was immersed for 10 seconds in themixed liquid and heated for 60 minutes at 120° C., and a PEDTpolymerization reaction was induced in the capacitor element to form asolid electrolyte layer. The capacitor element was inserted into acylindrical outer case with a closed end, and sealing rubber was mountedin the open-end portion and sealed with a tightening operation. Agingwas thereafter carried out for 120 minutes at 150° C. with an appliedvoltage of 5.2 V to form a solid electrolytic capacitor. The ratedvoltage of the solid electrolytic capacitor was 2.5 WV, and the ratedcapacity was 180 μF.

Comparative Example D1

After chemical repair, an electroconductive polymer was formed withoutimmersing the capacitor element in an aqueous solution of boric acid.Other conditions and steps were the same as Example D1.

Comparative Example D2

A separator containing 49 wt % of PET fiber as the main fiber was usedand a PET polymer was used as a binder. Other conditions and steps werethe same as Example D1.

Comparative Example D3

After chemical repair, the capacitor element was immersed in aqueoussolution of 5 wt % boric acid, and heating treatment was not performed.Other conditions and steps were the same as Example D1.

Comparative Example B1

The capacitor element was not immersed in an aqueous solution of boricacid or in a coupling agent, but other conditions and steps were thesame as Example D2.

[Comparison of the Results]

The initial characteristics and the number of occurrences of shortingfor 50 solid electrolytic capacitors in the working and ComparativeExamples obtained with the method described above were investigated andthe results shown in Table 3 were obtained. The non-defective units inwhich shorting did not occur were subjected to lead-free re-flowing at apeak temperature of 250° C. and held for 30 seconds at no less than 230°C. A surge test was thereafter performed such that charging anddischarging were carried out at 32.5 V in the course of 1,000 cycles ata temperature of 125° C. When the shorting voltage was measured, theresults shown in Table 3 were obtained.

When the initial characteristics of Example D2 and Comparative ExampleD4 obtained with the above-described method were investigated, theresults shown in Table 4 were obtained. TABLE 3 SHORTING INITIAL CHAR-NUMBER OF VOLTAGE ACTERISTICS OCCURENCES AFTER CAP ESR OF SHORTING SURGE(μF) (Ω/100 kHz) AFTER AGING TEST (V) EXAMPLE D1 15.3 0.045 0 52.5COMPARATIVE 15.0 0.050 6 51.0 EXAMPLE D1 COMPARATIVE 14.2 0.055 7 51.0EXAMPLE D2 COMPARATIVE 15.1 0.047 4 51.2 EXAMPLE D3

It is apparent from Table 3 that in Example D1, in which a separatorcontaining PVA polymer was used and an aqueous solution of boric acidwas added to the capacitor element, the initial characteristics, thenumber of occurrences of shorting after the aging step, and the shortingvoltage after a surge were all superior in comparison with theComparative Examples D1 to D3.

In contrast, the initial characteristics of Comparative Example D1, inwhich the boric acid treatment was not performed, and ComparativeExample D2, in which a separator that did not contain a compound with avinyl group, were poor in comparison with Example D1. ComparativeExample D3, in which heating treatment was not performed, exhibitedbetter results than Comparative Examples D1 or D2, but the results wereinferior in comparison with Example D1. TABLE 4 IMMERSION INITIAL CHAR-IMMERSION IN SILANE ACTERISTICS IN COUPLING CAP ESR BORIC ACID AGENT(μF) (Ω/100 kHz) EXAMPLE D2 IMMERSED IMMERSED 152 0.021 COMPARATIVE NOTNOT 132 0.023 EXAMPLE D4 IMMERSED IMMERSED

When Example D2, in which the capacitor element was immersed in acoupling agent, and Comparative Example D4, in which a coupling agentwas not used to impregnate the capacitor element, are examined in Table4, it is apparent that, in comparison with Comparative Example D4, theelectrostatic capacity increased by 1.15 times and the ESR was reducedby about 91% in Example D2 in which a coupling agent was added.

The shorting voltage increased 20 V when compared with the case in whicha separator that does not contain PVA as a binder is used.

(E) Fourth Embodiment

The present embodiment is a modified example of the third embodimentdescribed above. A capacitor element is formed using a separatorcontaining a compound with a vinyl group, and the additive that is addedto the capacitor element is dodecylbenzenesulfonic acid (DBS). It wasfound in this case as well that the same operation and effects as thethird embodiment can be obtained.

(E-1) Method for Manufacturing the Solid Electrolytic Capacitor

The method for manufacturing the solid electrolytic capacitor in thefourth embodiment is described below. That is, a cathode foil and ananode foil, with an oxidized film layer formed on the surface and aseparator containing a compound with a vinyl group interposedtherebetween, are wound together to form a capacitor element, and thecapacitor element is subjected to chemical repair. Next, a solution ofdodecylbenzenesulfonic acid is impregnated in the capacitor element, aconjugate composed of a compound with a vinyl group anddodecylbenzenesulfonic acid is formed, the capacitor element isthereafter immersed in a mixed liquid that was prepared by mixing apolymerizable monomer and an oxidizing agent together with apredetermined solvent, and a polymerization reaction involving theelectroconductive polymer is induced in the capacitor element to form asolid electrolyte layer. The capacitor element is then inserted in anouter case; sealing rubber is mounted in the open-end portion and sealedwith a tightening operation; and the unit is thereafter aged to form asolid electrolytic capacitor.

(E-2) Separator

The separator used in the present embodiment is preferably the sameseparator that is described in the section titled “(D-2) Separator” ofthe third embodiment.

(E-3) Dodecylbenzenesulfonic Acid (DBS)

Water, alcohol, or the like is preferably used as the solvent for thedodecylbenzenesulfonic acid used in the present invention. Theconcentration of the dodecylbenzene-sulfonic acid is preferably 0.1 wt %to 5 wt %, and even more preferably 0.2 wt % to 2 wt %. When theconcentration of the dodecylbenzenesulfonic acid solution is not in thisrange, the layer of the conjugate of DBS and PVA is thin, so theelectrostatic capacity, improvement in the ESR characteristics,improvement in the withstand voltage, and LC suppression effect are alldiminished. When the concentration of the dodecylbenzenesulfonic acidsolution exceeds this range, the layer of the conjugate of DBS and PVAis excessively thick, and the electroconductive characteristics of thelayer are reduced, so the electrostatic capacity and the ESRcharacteristics are also diminished.

(E-4) Method for Adding DBS to the Capacity Element

A method of immersing a capacitor element in the dodecylbenzenesulfonicacid solution, or a method of discharging the dodecylbenzenesulfonicacid solution over the capacitor element may be used for adding thedodecylbenzene-sulfonic acid to the capacitor element.

It was found that when dodecylbenzenesulfonic acid is added to thecapacitor element and heating treatment is performed thereafter, theinitial characteristics improve. It is thought that the reason for thisis that the compound with a vinyl group contained in the separatorelutes into the capacitor element and the hydrophobicity of the endgroups thereof increases, so the adhesiveness of the oxide film and thesolid electrolyte improves. The heating temperature is preferably 120 to250° C., and is more preferably 150 to 200° C. When the heatingtemperature is not in this range, the effect is reduced. This is thoughtto be due to the fact that when the heating temperature is less than120° C., the reaction that increases the hydrophobicity of the endgroups of the compound with a vinyl group does not adequately progress,and when the heating temperature exceeds 250° C., the thermaldegradation of the compound with a vinyl group takes place and theeffect is reduced.

(E-5) Timing for Adding DBS to the Capacity Element

The present inventors thoroughly researched the timing for adding thedodecylbenzenesulfonic acid to the capacitor element, and, as a result,it became apparent that the addition may be made at a stage prior to thestep for forming an electroconductive polymer, in the same manner as thethird embodiment, or at any other stage. In other words, the timing maybe prior to chemical repair, as described above, or the compound may beallowed to deposit on the electrode foils prior to forming the capacitorelement, and the methods (1) to (3) described below, for example, may beconsidered. The method in (1) corresponds to the manufacturing methoddescribed above.

The most advantageous of the methods (1) to (3) described below ismethod (1), in which an anode foil and cathode foil are wound while aseparator to which a compound with a vinyl group has been added isinterposed therebetween to form a capacitor element, the capacitorelement is subjected to chemical repair, the capacitor element is thenimpregnated with a solution of dodecylbenzenesulfonic acid, a conjugatecomposed of dodecylbenzenesulfonic acid and a compound with a vinylgroup is formed, and the polymerization reaction of theelectroconductive polymer is thereafter brought about in the capacitorelement to form a solid electrolyte layer.

Application of the methods described below may be made by substituting asolution of dodecylbenzenesulfonic acid for the solution of the boratecompound of FIGS. 2 to 4 shown in the third embodiment.

(1) After Chemical Repair . . . Refer to FIG. 2

This method is carried out in the following order: chemical conversion,capacitor element formation using a separator containing a compound witha vinyl group, chemical repair, immersion in a solution ofdodecylbenzenesulfonic acid, impregnation with a polymerizable monomerand an oxidizing agent, polymerization, insertion in an outer case,resin sealing, and aging.

(2) After Capacitor Element Formation, but Prior to Chemical Repair . .. Refer to FIG. 3

This method is carried out in the following order: chemical conversion,capacitor element formation using a separator containing a compound witha vinyl group, immersion in a solution of dodecylbenzenesulfonic acid,chemical repair, impregnation with a polymerizable monomer and anoxidizing agent, polymerization, insertion in an outer case, resinsealing, and aging.

(3) Prior to Capacitor Element Formation . . . Refer to FIG. 4

This method is carried out in the following order: chemical conversion,immersion of at least one of the electrode foils in a solution ofdodecylbenzenesulfonic acid (or application and drying thereafter),capacitor element formation using a separator containing a compound witha vinyl group, chemical repair, impregnation with a polymerizablemonomer and an oxidizing agent, polymerization, insertion in an outercase, resin sealing, and aging.

The concentration of the solution of dodecylbenzene-sulfonic acid,temperature, impregnation time, drying temperature, drying time, andother parameters in these methods are the same as described above.

(E-6) EDT and Oxidizing Agent

When EDT is used as the polymerizable monomer, an EDT monomer can beused as the EDT with which the capacitor element is impregnated, but amonomer solution in which EDT and a volatile solvent are mixed at avolume ratio of 1:0 to 1:3 may also be used.

Pentane or another hydrocarbon, tetrahydrofuran or another ether, ethylformate or another ester, acetone or another ketone, methanol or anotheralcohol, or acetonitrile or another nitrogen compound may be used as thevolatile solvent, but preferably used among these is methanol, ethanol,acetone, or the like.

Ferric p-toluenesulfonate dissolved in ethanol, or an aqueous solutionof periodic acid or iodic acid can be used as the oxidizing agent, butthe concentration of the oxidizing agent with respect to the solvent ispreferably 40 to 57 wt %, and is more preferably 45 to 57 wt %. The ESRdecreases with increased concentration of the oxidizing agent withrespect to the solvent. A volatile solvent used for the above-describedmonomer solutions may be used as the solvent for the oxidizing agent,and among these ethanol is advantageous. The reason that ethanol isadvantageous as the solvent for the oxidizing agent is believed to bethat the vapor pressure is low, so evaporation easily occurs, and theremaining amount is small.

(E-7) Reduced Pressure

It is even more preferable to reduce the pressure in the polymerizationstep. The reason for this is that when the pressure is reduced duringheat polymerization, residual matter can be evaporated away togetherwith polymerization. The pressure is preferably reduced to about 10 to360 mmHg.

(E-8) Impregnation Step

The time for impregnating the capacitor element with the mixed liquid isdetermined by the size of the capacitor element, but 5 seconds or moreis preferred for a capacitor element with a size of about φ 5×3 L, and10 seconds or more is preferred for a capacitor element with a size ofabout φ 9×5 L. Impregnation for 5 seconds is required at minimum. Thereare no drawbacks to the characteristics even if impregnation is carriedout over a long period of time.

It is advantageous to keep the pressure reduced after impregnation hasbeen performed in this manner. It is thought that the reason for this isthat the residual amount of volatile solvent is reduced. The reducedpressure conditions are the same as the reduced pressure conditions inthe above-described polymerization step.

(E-9) Chemical Conversion Solution for Chemical Repair

Ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or anotherphosphate-based chemical conversion solution; ammonium borate or anotherboric acid-based chemical conversion solution; or ammonium adipate oranother adipic acid-based chemical conversion solution may be used asthe chemical conversion solution for chemical repair, but preferablyused among these is ammonium dihydrogen phosphate. The immersion time ispreferably 5 to 120 minutes.

(E-10) Other Polymerizable Monomers

The polymerizable monomer used in the present invention may be, inaddition to the above-described EDT, a thiophene derivative, aniline,pyrrole, furan, acetylene, or a derivative thereof other than EDT, aslong as oxidation polymerization is carried out with a predeterminedoxidizing agent to form a polymerizable monomer. Substances with thestructural formula shown in FIG. 1 may be used for the thiophenederivative.

(E-11) Operation and Effect of the Fourth Embodiment

As described above, degradation of the withstand voltage characteristicsdue to lead-free reflow can be prevented, and the ratio of shortingoccurrence in the aging step can be considerably reduced by forming acapacitor element using a separator containing a compound with a vinylgroup, and adding dodecylbenzenesulfonic acid to the capacitor elementat a predetermined time.

It is thought that the reason that such effects can be obtained is thatby allowing the compound with a vinyl group contained in the separatorto elute into the capacitor element to form a conjugate withdodecylbenzenesulfonic acid or another hydrogen bond in the capacitorelement, and allowing this conjugate to deposit on the oxide film of theelectrode foils to form a layer, the adhesiveness of the oxide film andthe solid electrolyte improves, and because the withstand voltage ofthis layer is high, the withstand voltage of the capacitor is alsoimproved.

Allowing the compound with a vinyl group added to the separator to eluteas in the present invention is more advantageous than adding a compoundwith a vinyl group to the element after formation of the capacitorelement in that the conjugate with dodecylbenzenesulfonic acid uniformlydeposits on the oxide film.

In particular, it is thought that when PVA and dodecylbenzenesulfonicacid are used, a conjugate composed of an ester compound is formed, andthis ester compound deposits on the surface of the film withoutimmersion in a dielectric film, so adequate characteristics can beobtained because an electron-blocking film is formed.

As described above, it is thought that when heating treatment is carriedout after adding dodecylbenzenesulfonic acid, the bondingcharacteristics of the end group of the compound with a vinyl groupeluted from the separator with respect to the dielectric oxide layer andthe electroconductive polymer are improved, the initial characteristics,and the electrostatic capacity and ESR characteristics in particular,are enhanced, the withstand voltage is improved, and the LC is reduced.

(E-12) Examples Related to the Fourth Embodiment . . . Part 1

The invention of the fourth embodiment is described next on the basis ofExamples E1 to E4 and Comparative Example E1 manufactured in the mannerdescribed below.

Example E1

A solid electrolytic capacitor was fabricated as follows by using aseparator containing a PVA polymer as a binder and containing 49 wt % ofPET fiber as the main fiber. An electrode-forming device was connectedto the cathode foil and the anode foil on whose surface an oxide filmlayer had been formed, and both electrode foils were wound with theinterposed separator to form a capacitor element whose element shape was7φ×5 L. The capacitor element was immersed for 40 minutes in ammoniumdihydrogen phosphate, and chemical repair was carried out. After thechemical repair, the capacitor element was dried at 100° C., immersed inan aqueous solution of 0.1 wt % dodecylbenzenesulfonic acid, and heatedat 150° C.

EDT and an ethanol solution of 45% ferric p-toluenesulfonate were mixedin a container, and the capacitor element was immersed for 10 seconds inthe mixed liquid, held in a state of reduced pressure of about 250 mmHg,and subsequently heated for 60 minutes at 120° C. under the sameconditions. A PEDT polymerization reaction was induced in the capacitorelement to form a solid electrolyte layer.

The capacitor element was inserted into a cylindrical outer case with aclosed end, and sealing rubber was mounted in the open-end portion andsealed with a tightening operation. Aging was thereafter carried out for120 minutes at 150° C. with an applied voltage of 33 V to form a solidelectrolytic capacitor. The rated voltage of the solid electrolyticcapacitor was 25 WV, and the rated capacity was 22 μF.

Example E2

The concentration of the dodecylbenzenesulfonic acid solution was 0.5 wt%. A solid electrolytic capacitor was fabricated with the otherparameters the same as Example E1.

Example E3

The concentration of the dodecylbenzenesulfonic acid solution was 2.0 wt%. A solid electrolytic capacitor was fabricated with the otherparameters the same as Example E1.

Example E4

The concentration of the dodecylbenzenesulfonic acid solution was 5.0 wt%. A solid electrolytic capacitor was fabricated with the otherparameters the same as Example E1.

Comparative Example E1

Chemical repair was carried out without immersion in a solution ofdodecylbenzenesulfonic acid to form an electroconductive polymer. Otherconditions and steps were the same as Example E1.

[Comparison of the Results]

When the initial characteristics and the reflow characteristics of thesolid electrolytic capacitors of Examples E1 to E4 and ComparativeExample E1 obtained with the above-described method were investigated,the results shown in Table 5 were obtained. The reflow test was carriedout by subjecting the capacitors to lead-free re-flowing at a peaktemperature of 250° C. and keeping the capacitors for 30 seconds at noless than 230° C., and the leakage current was calculated. TABLE 5INITIAL DBS CHARACTERISTICS REFLOW CONCEN- WITHSTAND CHARACTER- TRATIONVOLTAGE LC ISTIC LC (%) (V) (μA) (μA) EXAMPLE E1 0.1 58 0.3 20 EXAMPLEE2 0.5 59 0.7 9 EXAMPLE E3 2.0 58 1.0 20 EXAMPLE E4 5.0 55 1.5 20COMPARATIVE 0 54 2.0 30 EXAMPLE E1

It is apparent from Table 5 that in Examples E1 to E4, in which aseparator containing PVA polymer was used and a dodecylbenzenesulfonicacid solution was added to the capacitor element, the withstand voltagewas 55 to 59 V, each resulting in an improvement in comparison with theComparative Example E1 (54 V). In particular, in the Examples E1 to E3,in which the DBS concentration was 0.1 to 2.0%, the withstand voltageexhibited a high value of 58 to 59 V.

The initial and after-reflow leakage currents decreased to 0.3 to 1.5 μAand 9 to 20 μA, respectively, while those for Comparative Example E1were 2 μA and 30 μA.

(E-13) Examples Related to the Fourth Embodiment . . . Part 2

The invention of the fourth embodiment is described next on the basis ofExample E5 and Comparative Example E2 manufactured in the mannerdescribed below.

Example E5

A solid electrolytic capacitor was fabricated as follows by using aseparator containing a PVA polymer as a binder and containing 49 wt % ofPET fiber as the main fiber. An electrode-forming device was connectedto the cathode foil and the anode foil on whose surface an oxide filmlayer had been formed, and both electrode foils were wound with theinterposed separator to form a capacitor element whose element shape was5φ×3 L. The capacitor element was immersed for 40 minutes in ammoniumdihydrogen phosphate, and chemical repair was carried out. After thechemical repair, the capacitor element was dried at 100° C., immersedfor three minutes at normal temperature in an aqueous solution of 0.5 wt% dodecylbenzenesulfonic acid, and heated at 150° C.

EDT and an ethanol solution of 45% ferric p-toluenesulfonate were mixedin a container, and the capacitor element was immersed for 10 seconds inthe mixed liquid, held in a state of reduced pressure of about 250 mmHg,and subsequently heated for 60 minutes at 120° C. under the sameconditions. A PEDT polymerization reaction was induced in the capacitorelement to form a solid electrolyte layer.

The capacitor element was inserted into a cylindrical outer case with aclosed end, and sealing rubber was mounted in the open-end portion andsealed with a tightening operation. Aging was thereafter carried out toform a solid electrolytic capacitor. The rated voltage of the solidelectrolytic capacitor was 6.3 WV, and the rated capacity was 180 μF.

Comparative Example E2

Chemical repair was carried out without immersion indodecylbenzenesulfonic acid solution to form an electroconductivepolymer. Other conditions and steps were the same as Example E5.

[Comparison of the Results]

When the initial characteristics of the solid electrolytic capacitors ofExample E5 and comparative example E2 obtained with the above-describedmethod were investigated, the results shown in Table 6 were obtained.TABLE 6 INITIAL DBS CHARACTERISTICS CONCENTRATION CAP ESR (%) (μF)(Ω/100 kHz) EXAMPLE E5 0.5 185 0.013 COMPARATIVE 0 180 0.015 EXAMPLE E2

It is apparent from Table 6 that in Example E5, in which a separatorcontaining PVA polymer was used and a dodecylbenzenesulfonic acidsolution was added to the capacitor element, the results were superiorin comparison with the Comparative Example E2.

(F) Fifth Embodiment

The present embodiment is a modified example of the third embodimentdescribed above. A capacitor element is formed using a separatorcontaining a compound with a vinyl group, and the additive that is addedto the capacitor element is sodium naphthalenesulfonate (NPS). It wasfound in this case as well that the same operation and effects as thethird embodiment can be obtained.

(F-1) Method for Manufacturing the Solid Electrolytic Capacitor

The method for manufacturing the solid electrolytic capacitor in thefifth embodiment is described below. That is, a cathode foil and ananode foil, with an oxidized film layer formed on the surface and aseparator containing a compound with a vinyl group interposedtherebetween, are wound together to form a capacitor element, and thecapacitor element is subjected to chemical repair. Next, a solution ofsodium naphthalenesulfonate is used to impregnate the capacitor element,a conjugate composed of a compound with a vinyl group and sodiumnaphthalenesulfonate is formed, the capacitor element is thereafterimmersed in a mixed liquid that was prepared by mixing a polymerizablemonomer and an oxidizing agent together with a predetermined solvent,and a polymerization reaction involving the electroconductive polymer isinduced in the capacitor element to form a solid electrolyte layer. Thecapacitor element is then inserted in an outer case; sealing rubber ismounted in the open-end portion and sealed with a tightening operation;and the unit is thereafter aged to form a solid electrolytic capacitor.

(F-2) Separator

The separator used in the present embodiment is preferably the sameseparator that is described in the section titled “(D-2) Separator” ofthe third embodiment.

(F-3) Sodium Naphthalenesulfonate (NPS)

Water, alcohol, or the like is preferably used as the solvent for thesodium naphthalenesulfonate used in the present invention. Theconcentration of the sodium naphthalenesulfonate is preferably 0.1 wt %to 5 wt %, and even more preferably 0.2 wt % to 2 wt %. When theconcentration of the sodium naphthalenesulfonate solution is not in thisrange, the layer of the conjugate of NPS and PVA is thin, so theelectrostatic capacity, improvement in the ESR characteristics,improvement in the withstand voltage, and LC suppression effect are alldiminished. When the concentration of the sodium naphthalenesulfonatesolution exceeds this range, the layer of the conjugate of NPS and PVAis excessively thick, and the electroconductive characteristics of thelayer are reduced, so the electrostatic capacity and the ESRcharacteristics are also diminished.

(F-4) Method for Adding NPS to the Capacity Element

A method of immersing a capacitor element in the sodiumnaphthalenesulfonate solution, or a method of discharging the sodiumnaphthalenesulfonate solution over the capacitor element may be used foradding the sodium naphthalene-sulfonate to the capacitor element.

It was found that when sodium naphthalenesulfonate is added to thecapacitor element and heating treatment is performed thereafter, theinitial characteristics improve. It is thought that the reason for thisis that the compound with a vinyl group contained in the separatorelutes into the capacitor element and the hydrophobicity of the endgroups thereof increases, so the adhesiveness of the oxide film and thesolid electrolyte improves. The heating temperature is preferably 120 to250° C., and is more preferably 150 to 200° C. When the heatingtemperature is not in this range, the effect is reduced. This is thoughtto be due to the fact that when the heating temperature is less than120° C., reaction that increases the hydrophobicity of the end groups ofthe compound with a vinyl group does not adequately progress, and whenthe heating temperature exceeds 250° C., the thermal degradation of thecompound with a vinyl group takes place and the effect is reduced.

(F-5) Timing for Adding NPS to the Capacity Element

The present inventors thoroughly researched the timing for adding thesodium naphthalenesulfonate to the capacitor element, and, as a result,it was apparent that the addition may be made at a stage prior to thestep for forming an electroconductive polymer, in the same manner as thethird embodiment, or at any other stage. In other words, the timing maybe prior to chemical repair, as described above, or the compound may beallowed to deposit on the electrode foils prior to forming the capacitorelement, and the methods (1) to (3) described below, for example, may beconsidered. The method in (1) corresponds to the manufacturing methoddescribed above.

The most advantageous of the methods (1) to (3) described below ismethod (1), in which an anode foil and cathode foil are wound while aseparator to which a compound with a vinyl group has been added isinterposed therebetween to form a capacitor element, the capacitorelement is subjected to chemical repair, the capacitor element is thenimpregnated with a solution of sodium naphthalenesulfonate, a conjugatecomposed of sodium naphthalenesulfonate and a compound with a vinylgroup is formed, and the polymerization reaction of theelectroconductive polymer is thereafter brought about in the capacitorelement to form a solid electrolyte layer.

Application of the methods described below may be made by substituting asolution of sodium naphthalenesulfonate for the solution of the boratecompound of FIGS. 2 to 4 shown in the third embodiment.

(1) After Chemical Repair . . . Refer to FIG. 2

This method is carried out in the following order: chemical conversion,capacitor element formation using a separator containing a compound witha vinyl group, chemical repair, immersion in a solution of sodiumnaphthalenesulfonate, impregnation with a polymerizable monomer and anoxidizing agent, polymerization, insertion in an outer case, resinsealing, and aging.

(2) After Capacitor Element Formation, but Prior to Chemical Repair . .. Refer to FIG. 3

This method is carried out in the following order: chemical conversion,capacitor element formation using a separator containing a compound witha vinyl group, immersion in a solution of sodium naphthalenesulfonate,chemical repair, impregnation with a polymerizable monomer and anoxidizing agent, polymerization, insertion in an outer case, resinsealing, and aging.

(3) Prior to Capacitor Element Formation . . . Refer to FIG. 4

This method is carried out in the following order: chemical conversion,immersion of at least one of the electrode foils in a solution of sodiumnaphthalenesulfonate (or application and drying thereafter), capacitorelement formation using a separator containing a compound with a vinylgroup, chemical repair, impregnation with a polymerizable monomer and anoxidizing agent, polymerization, insertion in an outer case, resinsealing, and aging.

The concentration of the solution of sodium naphthalenesulfonate,temperature, impregnation time, drying temperature, drying time, andother parameters in these methods are the same as described above.

(F-6) EDT and Oxidizing Agent

When EDT is used as the polymerizable monomer, an EDT monomer can beused as the EDT with which the capacitor element is impregnated, but amonomer solution in which EDT and a volatile solvent are mixed at avolume ratio of 1:0 to 1:3 may also be used.

Pentane or another hydrocarbon, tetrahydrofuran or another ether, ethylformate or another ester, acetone or another ketone, methanol or anotheralcohol, or acetonitrile or another nitrogen compound may be used as thevolatile solvent, but preferably used among these is methanol, ethanol,acetone, or the like.

Ferric p-toluenesulfonate dissolved in ethanol, or an aqueous solutionof periodic acid or iodic acid can be used as the oxidizing agent, butthe concentration of the oxidizing agent with respect to the solvent ispreferably 40 to 57 wt %, and is more preferably 45 to 57 wt %. The ESRdecreases with increased concentration of the oxidizing agent withrespect to the solvent. A volatile solvent used for the above-describedmonomer solutions may be used as the solvent for the oxidizing agent,and among these ethanol is advantageous. The reason that ethanol isadvantageous as the solvent for the oxidizing agent is believed to bethat the vapor pressure is low, so evaporation easily occurs, and theremaining amount is small.

(F-7) Reduced Pressure

It is even more preferable to reduce the pressure in the polymerizationstep. The reason for this is that when the pressure is reduced duringheat polymerization, residual matter can be evaporated away togetherwith polymerization. The pressure is preferably reduced to about 10 to360 mmHg.

(F-8) Impregnation Step

The time for impregnating the capacitor element with the mixed liquid isdetermined by the size of the capacitor element, but 5 seconds or moreis preferred for a capacitor element with a size of about φ 5×3 L, and10 seconds or more is preferred for a capacitor element with a size ofabout φ 9×5 L. Impregnation for 5 seconds is required at minimum. Thereare no drawbacks to the characteristics even if impregnation is carriedout over a long period of time.

It is advantageous to keep the pressure reduced after impregnation hasbeen performed in this manner. It is thought that the reason for this isthat the residual amount of volatile solvent is reduced. The reducedpressure conditions are the same as the reduced pressure conditions inthe above-described polymerization step.

(F-9) Chemical Conversion Solution for Chemical Repair

Ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or anotherphosphate-based chemical conversion solution; ammonium borate or anotherboric acid-based chemical conversion solution; or ammonium adipate oranother adipic acid-based chemical conversion solution may be used asthe chemical conversion solution for chemical repair, but preferablyused among these is ammonium dihydrogen phosphate. The immersion time ispreferably 5 to 120 minutes.

(F-10) Other Polymerizable Monomers

The polymerizable monomer used in the present invention may be, inaddition to the above-described EDT, a thiophene derivative, aniline,pyrrole, furan, acetylene, or a derivative thereof other than EDT, aslong as oxidation polymerization is carried out with a predeterminedoxidizing agent to form a polymerizable monomer. Substances with thestructural formula shown in FIG. 1 may be used for the thiophenederivative.

(F-11) Operation and Effect of the Fifth Embodiment

As described above, degradation of the withstand voltage characteristicsdue to lead-free reflow can be prevented, and the ratio of shortingoccurrence in the aging step can be considerably reduced by forming acapacitor element using a separator containing a compound with a vinylgroup, and adding sodium naphthalenesulfonate to the capacitor elementat a predetermined time.

It is thought that the reason that such effects can be obtained is thatby allowing the compound with a vinyl group contained in the separatorto elute into the capacitor element to form a conjugate with sodiumnaphthalenesulfonate or another hydrogen bond in the capacitor element,and allowing this conjugate to deposit on the oxide film of theelectrode foils to form a layer, the adhesiveness of the oxide film andthe solid electrolyte improves, and because the withstand voltage ofthis layer is high, the withstand voltage of the capacitor is alsoimproved.

Allowing the compound with a vinyl group added to the separator to eluteas in the present invention is more advantageous than adding a compoundwith a vinyl group to the element after formation of the capacitorelement in that the conjugate with sodium naphthalenesulfonate uniformlydeposits on the oxide film.

In particular, it is thought that when PVA and sodiumnaphthalenesulfonate are used, a conjugate composed of an ester compoundis formed, and this ester compound deposits on the surface of the filmwithout immersion in a dielectric film, so adequate characteristics canbe obtained because an electron-blocking film is formed.

As described above, it is thought that when heating treatment is carriedout after adding sodium naphthalene-sulfonate, the bondingcharacteristics of the end group of the compound with a vinyl groupeluted from the separator with respect to the dielectric oxide layer andthe electroconductive polymer are improved, the initial characteristics,and the electrostatic capacity and ESR characteristics in particular,are enhanced, the withstand voltage is improved, and the LC is reduced.

(F-12) Examples Related to the Fifth Embodiment

The invention of the fifth embodiment is described next on the basis ofExamples F1 to F4 and Comparative Example F1 manufactured in the mannerdescribed below.

Example F1

A solid electrolytic capacitor was fabricated as follows by using aseparator containing PVA polymer as a binder and containing 49 wt % ofPET fiber as the main fiber. An electrode-forming device was connectedto the cathode foil and the anode foil on whose surface an oxide filmlayer had been formed, and both electrode foils were wound with theinterposed separator to form a capacitor element whose element shape was7φ×5 L. The capacitor element was immersed for 40 minutes in ammoniumdihydrogen phosphate, and chemical repair was carried out. After thechemical repair, the capacitor element was dried at 100° C., immersedfor three minutes at normal temperature in an aqueous solution of 0.1-wt% sodium naphthalenesulfonate, and heated at 150° C.

EDT and an ethanol solution of 45% ferric p-toluene-sulfonate were mixedin a container, and the capacitor element was immersed for 10 seconds inthe mixed liquid, held in a state of reduced pressure of about 250 mmHg,and subsequently heated for 60 minutes at 120° C. under the sameconditions. A PEDT polymerization reaction was induced in the capacitorelement to form a solid electrolyte layer.

The capacitor element was inserted into a cylindrical outer case with aclosed end, and sealing rubber was mounted in the open-end portion andsealed with a tightening operation. Aging was thereafter carried out for120 minutes at 150° C. with an applied voltage of 33 V to form a solidelectrolytic capacitor. The rated voltage of the solid electrolyticcapacitor was 25 WV, and the rated capacity was 22 μF.

Example F2

The concentration of the sodium naphthalenesulfonate solution was 0.5 wt%. A solid electrolytic capacitor was fabricated with the otherparameters the same as Example F1.

Example F3

The concentration of the sodium naphthalenesulfonate solution was 2.0 wt%. A solid electrolytic capacitor was fabricated with the otherparameters the same as Example F1.

Example F4

The concentration of the sodium naphthalenesulfonate solution was 5.0 wt%. A solid electrolytic capacitor was fabricated with the otherparameters the same as Example F1.

Comparative Example F1

Chemical repair was carried out without immersion in a solution ofsodium naphthalenesulfonate to form an electroconductive polymer. Otherconditions and steps were the same as Example F1.

[Comparison of the Results]

When the initial characteristics and the reflow characteristics of thesolid electrolytic capacitors of Examples F1 to F4 and ComparativeExample F1 obtained with the above-described method were investigated,the results shown in Table 7 were obtained. The reflow test was carriedout by subjecting the capacitors to lead-free re-flowing at a peaktemperature of 250° C. and keeping the capacitors for 30 seconds at noless than 230° C., and the leakage current was calculated. TABLE 7INITIAL NPS CHARACTERISTICS REFLOW CONCEN- WITHSTAND CHARACTER- TRATIONVOLTAGE LC ISTIC LC (%) (V) (μA) (μA) EXAMPLE F1 0.1 57 0.2 20 EXAMPLEF2 0.5 57 0.3 10 EXAMPLE F3 2.0 57 0.5 10 EXAMPLE F4 5.0 55 0.1 8COMPARATIVE 0 54 2.0 30 EXAMPLE F1

It is apparent from Table 7 that in Examples F1 to F4, in which aseparator containing PVA polymer was used and a dodecylbenzenesulfonicacid solution was added to the capacitor element, the withstand voltagewas 55 to 57 V, each resulting in an improvement in comparison withComparative Example F1 (54 V) In particular, in the Examples F1 to F3,in which the NPS concentration was 0.1 to 2.0%, the withstand voltageexhibited a high value of 57 V.

The initial and after-reflow leakage currents decreased to 0.1 to 0.5 μAand 8 to 20 μA, respectively, while those for Comparative Example F1were 2 μA and 30 μA.

(G) Sixth Embodiment

The present inventors, as a result of thoroughgoing research to achievethe fourth object described above, which is to improve the withstandvoltage of a solid electrolytic capacitor and to inhibit LC fluctuationafter reflow, perfected the invention described in the sixth embodiment.Specifically, the inventors focused on the mechanism of electricalconductivity and studied electrical conductivity in electroconductivepolymers and other solid polymers.

Electron emission is commonly largely divided into two types: emissionby tunneling electrons and emission by electrons that pass over (passageof electrons that does not depend on film damage) the potential barrier,and it is possible that the leakage current in the solid electrolyticcapacitor is caused by electrons that pass over the potential barrier,rather than being a result of a shorted condition.

It is possible that the causes of an increase in LC after re-flowing aremechanical stress (physical stress) caused by the generation of gasduring re-flowing, and chemical stress (attack of the oxidizing agent,passage of electrons, or the like).

In other words, in a solid electrolytic capacitor with a higherwithstand voltage, shorting occurs when electrons increase in number, aflooded state is created, and a tunneling condition occurs. An increasein LC during reflow is due to the passage of electrons rather thaninsulation breakdown, so the common factor in both cases is electrons.Therefore, electrons can be inhibited in their ability to pass over thepotential barrier, and an increase in LC during re-flowing due to ahigher withstand voltage can be reduced by electron blocking.

Based on this knowledge, and as a result of thoroughgoing researchregarding a technique that can prevent electrons from passing over thepotential barrier, the present inventors discovered that adding acompound with a vinyl group to the separator, forming a capacitorelement using this separator, and adding polyimide silicon to thecapacitor element makes it possible to improve the withstand voltage ofthe product and to inhibit an increase in LC after re-flowing.

(G-1) Method for Manufacturing the Solid Electrolytic Capacitor

The method for manufacturing the solid electrolytic capacitor in thesixth embodiment is described below. That is, a cathode foil and ananode foil, with an oxidized film layer formed on the surface and aseparator containing a compound with a vinyl group interposedtherebetween, are wound together to form a capacitor element, and thecapacitor element is subjected to chemical repair. The capacitor elementis thereafter immersed in a solution in which polyimide silicon has beendissolved with a ketone-based solvent to a concentration of 10 wt % orless, preferably 1.5 to 9 wt %, and more preferably 5 to 8 wt %. Thecapacitor element is removed, the solvent is evaporated at 40 to 100°C., and the capacitor element is then subjected to heat treatment at 150to 200° C.

Next, the capacitor element is immersed in a mixed liquid of apolymerizable monomer and an oxidizing agent, and a polymerizationreaction involving the electroconductive polymer is induced in thecapacitor element to form a solid electrolyte layer. The capacitorelement is then housed in an outer case, and the open-end portion issealed with sealing rubber to form a solid electrolytic capacitor.

(G-2) Polyimide Silicon

The solvent that is used to dissolve polyimide silicon is preferably aketone-based solvent which has good solubility for polyimide silicon,and cyclohexanone, acetone, methyl ethyl ketone, or the like may beused.

The concentration of the polyimide silicon is preferably 10 wt % orless, is more preferably 1.5 to 9 wt %, and is even more preferably 5 to8 wt %. When the concentration of the polyimide silicon is less thanthis range, the withstand voltage is insufficient; and when theconcentration exceeds this range the electrostatic capacity decreases.

(G-3) Separator

The separator used in the present embodiment is preferably the sameseparator that is described in the section titled “(D-2) Separator” ofthe third embodiment.

(G-4) EDT and Oxidizing Agent

When EDT is used as the polymerizable monomer, an EDT monomer can beused as the EDT with which the capacitor element is impregnated, but amonomer solution in which EDT and a volatile solvent are mixed at avolume ratio of 1:0 to 1:3 may also be used.

Pentane or another hydrocarbon, tetrahydrofuran or another ether, ethylformate or another ester, acetone or another ketone, methanol or anotheralcohol, or acetonitrile or another nitrogen compound may be used as thevolatile solvent, but preferably used among these is methanol, ethanol,acetone, or the like.

Ferric p-toluenesulfonate dissolved in ethanol, or an aqueous solutionof periodic acid or iodic acid can be used as the oxidizing agent, butthe concentration of the oxidizing agent with respect to the solvent ispreferably 40 to 65 wt %, and is more preferably 45 to 57 wt %. The ESRdecreases with increased concentration of the oxidizing agent withrespect to the solvent. A volatile solvent used for the above-describedmonomer solutions may be used as the solvent for the oxidizing agent,and among these ethanol is advantageous. The reason that ethanol isadvantageous as the solvent for the oxidizing agent is believed to bethat the vapor pressure is low, so evaporation easily occurs, and theremaining amount is small.

(G-5) Chemical Conversion Solution for Chemical Repair

Ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or anotherphosphate-based chemical conversion solution; ammonium borate or anotherboric acid-based chemical conversion solution; or ammonium adipate oranother adipic acid-based chemical conversion solution may be used asthe chemical conversion solution for chemical repair, but preferablyused among these is ammonium dihydrogen phosphate. The immersion time ispreferably 5 to 120 minutes.

(G-6) Other Polymerizable Monomers

The polymerizable monomer used in the present invention may be, inaddition to the above-described EDT, a thiophene derivative, aniline,pyrrole, furan, acetylene, or a derivative thereof other than EDT, aslong as oxidation polymerization is carried out with a predeterminedoxidizing agent to form a polymerizable monomer. Substances with thestructural formula shown in FIG. 1 may be used for the thiophenederivative.

(G-7) Operation and Effect of the Sixth Embodiment

Following are the possible reasons that withstand voltage can beimproved and an inhibiting effect on LC fluctuation can be obtainedafter re-flowing with the configuration of the present invention.

In other words, it is possible that a film (hereinafter referred to as“electron-blocking layer) that prevents electrons form passing over thepotential barrier and that comprises a PVA layer and a polyimide siliconlayer is formed on the surface of the oxide film by immersing thecapacitor element in a polyimide silicon solution after chemical repair.

This electron-blocking layer improves the withstand voltage, preventsthe attack of the oxidizing agent on the foil, and reduces the initialLC. The effect of inhibiting an increase in LC during reflow can beobtained with a covering of Tabcoat. It is possible to control thewithstand voltage by making the electrostatic capacity and ESR largelyunaffected by other factors and controlling the thickness of thiselectron-blocking layer. The VF of foil currently used can be reduced,so solid electrolytic capacitors can be made smaller, the capacityincreased, and other beneficial effects obtained.

(G-8) Examples Related to the Sixth Embodiment

The present invention will now be described in greater detail on thebasis of Examples and a conventional example manufactured in the mannerdescribed below.

Example G1

A solid electrolytic capacitor was fabricated as follows by using aseparator containing PVA polymer as a binder and containing 49 wt % ofPET fiber as the main fiber. An electrode-forming device was connectedto the cathode foil and the anode foil on whose surface an oxide filmlayer had been formed, and both electrode foils were wound with theinterposed separator to form a capacitor element whose element shape was7φ×5 L. The capacitor element was immersed for 40 minutes in ammoniumdihydrogen phosphate, and chemical repair was carried out. The capacitorelement was then immersed in a 2 wt % cyclohexanone solution ofpolyimide silicon, removed, and thereafter heated for one hour at 170°C.

Next, EDT and an ethanol solution of 40 wt % ferric p-toluenesulfonatewere mixed in a container, the solution was injected so as to achieve aweight ratio of 1:3 to prepare a mixed liquid, and the capacitor elementwas immersed for 10 seconds in the mixed liquid to impregnate EDT and anoxidizing agent in the capacitor element. The capacitor element was thenleft for one hour in a thermostat at 120° C., and a PEDT polymerizationreaction was induced in the capacitor element to form a solidelectrolyte layer. The capacitor element was thereafter housed in acylindrical aluminum case with a closed end, sealed with sealing rubberto form a solid electrolytic capacitor.

Example G2

A capacitor element was immersed in 6 wt % cyclohexanone solution ofpolyimide silicon, then removed, and subjected to heat treatment for onehour at 170° C. A solid electrolytic capacitor was fabricated with theother parameters the same as Example G1.

Example G3

A capacitor element was immersed in 10 wt % cyclohexanone solution ofpolyimide silicon, then removed, and subjected to heat treatment for onehour at 170° C. A solid electrolytic capacitor was fabricated with theother parameters the same as Example G1.

Conventional Example G1

The capacitor element was not immersed in a cyclohexanone solution ofpolyimide silicon, and a solid electrolytic capacitor was fabricatedwith the other parameters the same as Example G1

[Comparison of the Results]

When the electrical characteristics of Examples G1 to G3 andconventional example G1 obtained with the above-described method wereinvestigated, the results shown in Table 8 were obtained. The decreasein initial capacity (ΔCap) is shown as a percent of a blank(conventional example). TABLE 8 INITIAL CONTENT CHARACTERISTICS LC (μA)OF ADDITIVE WITHSTAND ESR Δ CAP AFTER (wt %) VOLTAGE (V) (mΩ) (%)INITIAL REFLOW EXAMPLE G1 2.0 66 25.0 0 0.1 5 EXAMPLE G2 6.0 74 26.0 −180.1 0.9 EXAMPLE G3 10.0 75 26.5 −30 0.1 0.9 COMPARATIVE — 58 25.0 0 0.5150 EXAMPLE G1

It is apparent from Table 8 that in all the Examples E1 to E3, in whichthe capacitor element was immersed in a polyimide silicon solution afterchemical repair, the withstand voltage was improved, and the LC afterre-flowing was considerably reduced in comparison with the conventionalexample G1.

INDUSTRIAL APPLICABILITY

In accordance with the invention described in the first embodiment, asolid electrolytic capacitor and a manufacturing method thereof can beprovided in which it is possible to reduce the ESR and to improve theelectrostatic capacity.

In accordance with the invention described in the second embodiment, asolid electrolytic capacitor and a manufacturing method thereof can beprovided in which it is possible to reduce the ESR and to improve theelectrostatic capacity and withstand voltage by using a separatorcontaining 10 wt % or more of a compound with a vinyl group as a binder,and adding one or two compounds selected from acetylene diol anddimethyl lauryl amine oxide to the capacitor element prior toimpregnation with a polymerizable monomer and an oxidizing agent.

In accordance with the invention described in the fourth to fifthembodiments, a solid electrolytic capacitor with adequate initialcharacteristics can be provided in which a degradation of the withstandvoltage characteristics due to re-flowing can be prevented, and theyield when manufacturing a high withstand voltage product can improved,and also to provide a method for manufacturing this capacitor.

In accordance with the invention described in the sixth embodiment, asolid electrolytic capacitor and a manufacturing method thereof can beprovided in which the withstand voltage is improved and LC fluctuationafter re-flowing is inhibited.

1.-28. (canceled)
 29. A solid electrolytic capacitor comprising a capacitor element having an anode foil and a cathode foil wound with a separator interposed therebetween, the capacitor element being provided with a solid electrolyte layer of an electroconductive polymer by impregnating the capacitor element with a polymerizable monomer and an oxidizing agent, wherein a compound with a vinyl group is added to the separator, and the capacitor element wound using this separator is immersed in a polyimide silicon solution to form a film composed of polyimide silicon and a compound with a vinyl group on the surface of an oxide film.
 30. The solid electrolytic capacitor according to claim 29, characterized in that the polymerizable monomer is a thiophene derivative.
 31. The solid electrolytic capacitor according to claim 30, characterized in that the thiophene derivative is 3,4-ethylene dioxythiophene.
 32. The solid electrolytic capacitor according to claim 29, characterized in that the compound with a vinyl group is polyvinyl alcohol.
 33. The solid electrolytic capacitor according to claim 32, characterized in that the polymerizable monomer is a thiophene derivative.
 34. The solid electrolytic capacitor according to claim 33, characterized in that the thiophene derivative is 3,4-ethylene dioxythiophene.
 35. A solid electrolytic capacitor comprising a capacitor element having an anode foil and a cathode foil wound with a separator interposed therebetween, the capacitor element being provided with a solid electrolyte layer of an electroconductive polymer by impregnating the capacitor element with a polymerizable monomer and an oxidizing agent, wherein a compound with a vinyl group is added to the separator, and the capacitor element wound using this separator is immersed in a polyimide silicon solution to form, on the surface of an oxide film, a film consisting of two layers, one of which is of a compound with a vinyl group and the other of which is formed thereon of polyimide silicon.
 36. The solid electrolytic capacitor according to claim 35, characterized in that the polymerizable monomer is a thiophene derivative.
 37. The solid electrolytic capacitor according to claim 36, characterized in that the thiophene derivative is 3,4-ethylene dioxythiophene.
 38. The solid electrolytic capacitor according to claim 35, characterized in that the compound with a vinyl group is polyvinyl alcohol.
 39. The solid electrolytic capacitor according to claim 38, characterized in that the polymerizable monomer is a thiophene derivative.
 40. The solid electrolytic capacitor according to claim 39, characterized in that the thiophene derivative is 3,4-ethylene dioxythiophene.
 41. A method for manufacturing a solid electrolytic capacitor comprising the steps of: winding an anode foil and a cathode foil with a separator composed of a compound with a vinyl group interposed therebetween to form a capacitor element; immersing the capacitor element with a polyimide silicon solution to form a film composed of the polyimide silicon and the compound with a vinyl group on the surface of an oxide film; and impregnating the capacitor element with a polymerizable monomer and an oxidizing agent to form a solid electrolyte layer of an electroconductive polymer.
 42. The method for manufacturing a solid electrolytic capacitor according to claim 41, characterized in that the polymerizable monomer is a thiophene derivative.
 43. The method for manufacturing a solid electrolytic capacitor according to claim 41, characterized in that the thiophene derivative is 3,4-ethylene dioxythiophene.
 44. The method for manufacturing a solid electrolytic capacitor according to claim 44, characterized in that the compound with a vinyl group is polyvinyl alcohol.
 45. The method for manufacturing a solid electrolytic capacitor according to claim 44, characterized in that the polymerizable monomer is a thiophene derivative.
 46. The method for manufacturing a solid electrolytic capacitor according to claim 45, characterized in that the thiophene derivative is 3,4-ethylene dioxythiophene.
 47. A method for manufacturing a solid electrolytic capacitor comprising the steps of: winding an anode foil and a cathode foil with a separator composed of a compound with a vinyl group interposed therebetween to form a capacitor element; immersing the capacitor element with a polyimide silicon solution to form a film consisting of two layers, one of which is of the compound with a vinyl group and the other of which is formed thereon of the polyimide silicon, on the surface of an oxide film; and impregnating the capacitor element with a polymerizable monomer and an oxidizing agent to form a solid electrolyte layer of an electroconductive polymer.
 48. The method for manufacturing a solid electrolytic capacitor according to claim 47, characterized in that the polymerizable monomer is a thiophene derivative.
 49. The method for manufacturing a solid electrolytic capacitor according to claim 48, characterized in that the thiophene derivative is 3,4-ethylene dioxythiophene.
 50. The method for manufacturing a solid electrolytic capacitor according to claim 47, characterized in that the compound with a vinyl group is polyvinyl alcohol.
 51. The method for manufacturing a solid electrolytic capacitor according to claim 50, characterized in that the polymerizable monomer is a thiophene derivative.
 52. The method for manufacturing a solid electrolytic capacitor according to claim 51, characterized in that the thiophene derivative is 3,4-ethylene dioxythiophene. 